1. Field of the Invention
The present invention relates to a driving circuit to supply a controlled current to a load.
2. Description of the Related Art
Driving circuits employing semiconductor elements such as FETs (Field Effect Transistors) to supply current to load and drive the load are used in various fields. An example of a device employing such a driving circuit is a linear solenoid. The linear solenoid applies voltage to an excitation coil so as to generate magnetic force that directly linearly drives a movable iron core. The linear solenoid is capable of linearly controlling hydraulic pressure according to a control current from the driving circuit, and therefore, is used in the field of car electronics to control, for example, a braking hydraulic pressure and an automatic transmission.
FIG. 1 is a block diagram illustrating a driving circuit of a load according to a related art. The driving circuit includes a detection resistor 1, the load 2, a differential amplifier 4, an integrator 5, an output semiconductor element 8, an A/D converter 10, a controller 11, a PWM (Pulse Width Modulation) adjuster 12, a driver 13, and a power source 15. The driving circuit functions to supply a current instructed by the controller 11 to the load 2.
The detection resistor 1 is arranged upstream from the load 2, to receive a current Io passing from the power source 15 through the output semiconductor element 8 to the load 2 and convert the current Io into a voltage. The detection resistor 1 may be made of a semiconductor so that the detection resistor 1 is arranged on a single chip together with the output semiconductor element 8 and other devices, to reduce the cost and size of the driving circuit.
The differential amplifier 4 includes an operational amplifier 40 and resistors 41 and 42. When a current passes through the detection resistor 1 to the load 2, the differential amplifier 4 amplifies a voltage across the detection resistor 1. The amplified voltage from the differential amplifier 4 is indicative of the magnitude of the current passing through the load 2 and is used to control a current of the load 2.
The integrator 5 includes a resistor 50 and a capacitor 51, to smooth the voltage from the differential amplifier 4. Namely, the integrator 5 provides a voltage corresponding to an average of current passing through the load 2.
The output semiconductor element 8 is generally an PET and is connected in series in a power supply path from the power source 15 to the load 2. According to a drive voltage applied by the driver 13 to a gate of the output semiconductor element 8, the output semiconductor element 8 turns on/off (PWM control) to control the current Io to the load 2. The output semiconductor element 8 may be a bipolar transistor.
The A/D converter 10 compares an output voltage Vadc from the integrator 5 with a reference voltage Vr from a reference voltage source 18. The reference voltage Vr is stable with respect to temperature. The A/D converter 10 outputs, based on the comparison, a digital detected current value corresponding to the current Io to the load 2. Generally, the reference voltage Vr is set to be higher than the output voltage Vadc from the integrator 5.
The controller 11 is, for example, a CPU to calculate a current value necessary for making the load 2 perform an objective operation and output an instructed current value.
The PWM adjuster 12 compares the instructed current value with the detected current value from the A/D converter 10 and outputs a PWM signal to equalize a current to be passing through the load 2 with the instructed current value.
According to the PWM signal, the driver 13 conducts ON/OFF control of the output semiconductor element 8 to pass the instructed current through the load 2.
In this way, the driving circuit illustrated in FIG. 1 having the above-mentioned configuration conducts feedback control so that a current to the load 2 agrees with an instructed current value from the controller 11.
Another related art disclosed in Japanese Unexamined Patent Application Publication No. 2000-114039 is a linear solenoid failure detecting apparatus that quickly detects a short to ground in a wiring system or coil of a linear solenoid and prevents the deterioration or breakage of transistors and resistance elements in a driving stage. This apparatus includes the linear solenoid to obtain a displacement amount according to the magnitude of a driving current, a driver to control the driving current, a current detector to output a value corresponding to the driving current, an integrator having a resistance element and capacitor to integrate output values from the current detector, and an electric element that is connected in parallel with the resistance element of the integrator, to follow a rise of an output from the current detector and add a value corresponding to the output value from the current detector to an output value from the integrator.
If a short to ground occurs in the linear solenoid, the output value from the current detector shows a steep rise, which is instantaneously reflected in the output value from the integrator through the electric element (diode) connected in parallel with the resistance element. As results, a short-to-ground-failure detecting time in which the output voltage from the integrator exceeds a failure testing voltage becomes very short, to prevent the deterioration or breakage of the transistors and resistance elements in the driving stage of the linear solenoid.